Wednesday, December 19, 2007

How High?

In a world full of absurdist concepts, I’ve got one that won’t make you blink: Your bicycle is on your shoulder and you don’t live on a second story walk-up. You are running through a farm pasture, have not committed a crime and your bicycle works perfectly well.

Sounds like a great time, huh? Such is our love of cyclocross. There’s something in this equation that doesn’t quite add up, and it’s not that we carry our bicycles. It comes when we put that bike back down and willingly lob our asses skyward. Whether we have dreams of procreation or not, the delicate business means landing sidesaddle in proximity to a collection of biology known to us primarily for its ability to remind us of what NOT to do, the event is meant only to speed our return to the pedals. Click, click—and we’re off again.

Years ago the process was a little more involved. You had to hit each pedal with your foot, flip it over and jam your foot back in. Because races took place on grass, the bicycle’s bottom bracket had to be much higher than that of a traditional road bike so the toe clips wouldn’t drag in the grass—this is a detail the Frogs figured out in the 1950s.

Fast forward to, oh say, now. Swing the right knee skyward and once safely aboard, the feet go straight into the pedal stroke, no flipping over of the pedals.

So why are bottom bracket heights on cyclocross bikes still on average 2cm higher than those on road bikes?

It wouldn’t be a cause for concern were it not for this little detail. Name another cycling event where the rider makes tighter turns? Add to that the fact that these oh-so-tight turns are conducted aboard bikes with 700C wheels and it’s fair to ask the question: What can we do to make this bike easier to turn?

The answer is simple: Make it easier to lean the bike over to carve a tight turn. Okay, so how do you do that? Simple. Lower the bottom bracket. Drop the center of gravity of the bicycle and leaning the bicycle into a turn becomes a good bit easier.

How much could it be dropped? It’s hard to say; there hasn’t been much experimentation with this. Pedaling through corners doesn’t happen to the same degree it does in crits, so dragging a pedal through the dirt isn’t a big concern.

To illustrate the point, let’s consider an example in extreme. Say you’re driving down a twisty road. Would you rather take the twists and turns in a Ford Expedition or a Mini Cooper? My preference would be for the Mini Cooper, with a center of gravity lower than most Congressional standards, it can turn circles around the SUV. Put another way, I’d rather run a steeplechase barefoot than on stilts.

I went to the trouble to build, with the help of Toby Stanton of Hot Tubes, a ‘cross bike with a low bottom bracket. In Part II I’ll describe the process of building the bike and racing it at ‘Cross Nat’s and since.

35 comments:

yeah, you would think it'd be best to be in the mini cooper...but if you throw some big rocks, roots, stumps, etc in the middle of those twisty turns you quickly realize a subaru outback is a much better choice because of its ground clearance.

i went through the exact opposite, coming from a bike with too low of a bb that caused me to always clip a pedal to now having a bike with the right balance of high bb and adept handling.

I like your thinking, and while where at it a few other things that don't seem to make sense to me about cross bikes. Considering the cross races are infamous for being held, in the wettest, snowiest, muddies conditions you would think things like disc brakes and internal gearing would make sense as well how about a cross bike with some avid disc brakes, a rohloff hub and a track crank up front? I think it could be pretty sick, even if you couldn't race on it, it would be a really awesome commuter

I've heard it argued that a higher BB allows you to pedal through tight low speed turns where the the verb "to carve" is less operative than the verb "to slog." I would think this would be especially important in mud, where you're nearly always on the edge of losing momentum, partuclarly coming out of rutted corners.

Also, I wonder if trail isn't a neglected factor in 'cross bike handling. Lower trail geometries are getting a lot of attention from randonneurs because of their handling characteristics at low speeds. Might apply to cross as well.

Lower BB's will always be better for racing. I'm not talking any Jungle Cross with "Big rocks, roots,and stumps" If thats your think Ride a SALSA or a MTB. Low, light and fast. That would be my dream cross bike. T-

Very nice once again! But I'm not sold on BB height as being the deciding factor. I'm thinking a higher BB with less trail will make a bike turn easier. Someone will have to do some science to really sort this out.Craig

I tried to tell this to the design gurus at C'dale 5 years ago...they gave me the 1,000 yard stare. Then Tim Jiohnson siad lower the BB and they thought they had heard the voice of God. Check the bb height on their bikes.

What no one has mentioned.....which I think is worth mentioning.....is the fact that BB drop, when comparing from bike to bike, is the same no matter the tire radius.

BB height, however, changes drastically with tire choice. (And therefore center of gravity) And we know how many cross tyres are out there. Look at the review CN did on Wellens' bike. The dude is running like 32mm slicks!!!!! When he puts his "mud" tyres on, I'm sure his BB height is jacked by at least 1 cm.

And...once BB height changes, let's say it's getting higher off the ground for this example, that "raises the bar" of how high one needs to jump to get back on the bike. The saddle height from BB center to top of saddle remains the same from a 5.8 cm BB drop to a 7.0 BB drop.

Higher BB allows you to peddle through the corners allowing you to keep the momentum high.This was proven on crit bike and mountain bikes (when the races had actual single track)The ability to peddle in a corner can be a huge advantage higher lean angle are a good side effect too as they can block out more space.

BTW the use of a scientific (atmo) term is not cool if you don't know what it is.

The mini cooper analogy isn't entirely correct. On a car, it isn't the cg height that matters so much as the cg relative to track width. All else being equal, lowering the cg on a bike does nothing to the lean angle. Whether leaning on a taller bike feels like being on stilts or not depends entirely on the front end geometry.

retro grouch, just curious...have you tried them side by side? I have, on the same courses even... the larger bb drop trumped the 'pedalling through corners' everytime. I rode a 55mm bb drop for 5 years. It only took me a few races to see the 70mm bb drop was the clear winner. Oh yes...same fork rake on both bikes and nearly the same front end geo. 72.7 vs 72.5.

The history of building bicycles and parts has always shifted one direction to the other and then back again. It's a "cycle". Higher, lower, steeper, more or less are always "better" until it's the norm and then suddenly an old idea becomes better again.

1. As much as the temptation appears, I would refrain from using multi-track analogies for single track vehicles. Not a bad observation though.Single track vehicles such as bicycles or motorcycles lean into a turn to counter centrifugal forces, while multi-track vehicles corner flat or roll slightly out of the turn with significant load transfers to the outside wheels.

The dynamics are different.

2. Nice thoughts to a better cross bike. Such a bike could also be ridden on road. Why make a separate bike for everything right? Its easy for companies to cash in on this aspect.

3. Your idea seems to be prior knowledge to some good framebuilders. For example, Waterford makes/used to make cyclocross bikes with low BB drop. Not sure if they still do, but you might want to check.

4. Since I like to be very analytical in nature, no peer reviewed literature has talked about the greatest effect on CG on a bicycle. I mean, why does everyone think BB drop makes the most difference? I'd like someone to quantify the % drop in CG with, say, a cm of BB drop. And how is a compact geometry to lower CG compared with BB drop (assuming the compact bike will be made decent enough to shoulder)?

5. A good design in this area will think about proper trail, wheelbase, stability and stand over height.

6. If you don't want to worry about all these, I would just plead with the course designers to make more forgiving turns. Lol...

If only the rider's weight was in the bottom bracket. Instead it is about 60cm higher than that, where the saddle is located. BB height has little to do with center of gravity. However, a higher bottom gives greater clearance and a stiffer BB (shorter chainstays).

Anonymous 2:32: There is a valid connection between bb height and cg. If the bb gets raised or lowered, the saddle needs to move the same amount. Compact geometry on the other hand really doesn't have an effect. A top tube only weighs about 250g, compared to at least 79,000 grams for the rest of the bike + rider. Switching to a compact frame (moving the center of the top tube down 5cm) only moves the effective cg 0.15mm.

to get to the bottom of the BB height debate for cross bikes you need to go back to the roots of the sport. Most cross bikes are still roughley based on the old Alan geometry that was unchanged for years. in those years they used toeclips, and needed to keep them out of the mud, and THAT was the main factor for the raising of the BB. As well as that they were using 28c tires back then. Now we all know that bike makers will listen to their PRO riders in regard to changes in geometry, but when a rider has been winning on the same bike for years the last thing they want is to change it. you can get used to anything, but it takes time to learn(or unlearn) a bike's handling style. As for digging in a pedal once in a while I would contend that if you are pushing hard enough you SHOULD tap the pedal into the ground now and then. The best crit racers in the country do. Next time you are at a PRO crit take a look at some of the top guy's pedals, unless they just replaced them they will have scuff marks from hitting the ground while pedaling through corners. these days though, with no toeclips(thank god) and 32 and 34c tires, I want my BB low as hell. I really want to hear more about trail though

2. "The answer is simple: Make it easier to lean the bike over to carve a tight turn. Okay, so how do you do that? Simple. Lower the bottom bracket. Drop the center of gravity of the bicycle and leaning the bicycle into a turn becomes a good bit easier" .

Freddy, I'll be glad if you pointed out any sources of this information. True it happens in motorcycles (contrary to popular beliefs, some motorbikes are built with a higher engine position and higher CG for better leaning. Who do you believe?), but for a cyclocross application, with speeds much much lesser than those reached by motorbikes, I'll be very surprised to see if a cm or even two of BB drop will every do anything to leaning that is noticeable.

3. What leaning depends on more is the coefficient of friction of the ground, and the turning radius. Those two factors seem to overwhelm any other minute details.

4. Its also nice to keep the hands on the hoods while turning, that'll probably centralize CG (if you're worried about it) instead of it being more towards the rear. It could result in a stable turn.

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I spent 20 years of my life working in the bicycle industry, turning wrenches and selling bikes for some of the industry's best shops. I have extensive experience designing and constructing frames in both steel and titanium and have performed thousands of bike fits. I am passionate about bicycles in all forms. The bicycle provides me with physical and mental health and taps me into a social pipeline that allows me to share my passion with others. I ride as often as possible and love the flow of a hard group ride. Check back for musings about all things road cycling and, especially, the Spring Classics. The devil is in the details and I am an expert in the useless minutia that makes up our discipline.